The invention relates to new, aqueous, hydroxy-functional polyester dispersions containing urethane groups, to a process for preparing them, and to their use in combination with crosslinker resins for preparing paints, coatings and adhesives.
Aqueous polyester dispersions for high-solids baking varnishes are known, as disclosed for example in EP-A 498 156, which, as an essential constituent, include polyester resins containing urethane, carboxyl and hydroxyl groups. The polyester resins are reaction products of 77% to 89% by weight of a branched polyester polyol, 2.5% to 4.5% by weight of a bis(hydroxymethyl)alkanecarboxylic acid and 9% to 19% by weight of a diisocyanate component of which at least 50% is composed of hexamethylene diisocyanate. These products are suitable for preparing high-solids paints, but in certain applications no longer satisfy the requirements in terms of application reliability and of the resulting coating properties, particularly in terms of the paint flow and the appearance of surface defects. In order to bring the application reliability more into line with practical requirements, it is frequently necessary with these systems to use relatively large amounts of volatile organic solvents, which is undesirable on economic and environmental grounds, and does not always lead to satisfactory results.
Aqueous binder combinations comprising a polyester dispersion containing urethane, hydroxyl and carboxylate groups and a polyisocyanate component dispersed in the dispersion and containing free isocyanate groups are described, for example, in EP-A 496 205. The polyester dispersions of EP-A 496 205 contain, relatively high fractions of carboxyl and/or carboxylate groups (acid number 10 to 45 mg KOH/g substance), to allow the polyisocyanate component to be stably dispersed and to allow homogeneous, clear coatings. The application reliability and the film optical properties of the binder combinations of EP-A 496 205, however, frequently no longer satisfy the exacting present-day requirements in terms of flow, achievable coat thickness, and the resistance properties of the coating with respect to exposure to water or solvent.
EP-A 469 389 discloses aqueous 2-component polyurethane coating compositions based on aqueous polyurethane dispersions having a urethane and/or urea group content (—NH—COO—) of 9% to 20% and on water-dispersible polyisocyanate crosslinkers containing hydrophilic anionic and ethylene oxide groups. These products contain relatively large amounts of organic solvents and are suitable for preparing reactive 2-component coatings which can be cured at room temperature and have a narrowly restricted pot life, but not for storage-stable baking varnishes which can be processed as one-component systems. Owing to the high fractions of hydrophilic groups, the resistance properties and also the film optical properties are not always in accordance with the requirements.
DE-A 3 345 448 describes aqueous polyurethane baking varnishes comprising polyurethane prepolymers containing carboxylate groups and blocked isocyanate groups, and also polyhydroxy compounds which contain carboxylate groups and are based on polyether- and/or polyester-urethane prepolymers. These products contain relatively large amounts of organic solvents and are notable in particular for their ready pigmentability. In respect of application reliability, the flow of the coatings, and the achievable defect-free coat thickness, however, there exists need for improvement. In this system the polyethers used in addition may have an adverse impact on the weathering properties of the coatings. Likewise, as a result of the presence of carboxylate groups in both components of the baking varnishes, the hydrophilicity of the coatings is increased.
DE-A 3 936 288 describes water-dispersible binder combinations for baking surfaces, which comprise urethane-modified polyester resins containing carboxylate groups, among others. The resins have acid number of 15 to 36 mg KOH/g substance and are reaction products of 72% to 80% by weight of polyester polyol, 4% to 7% by weight of 2,2-bis(hydroxymethyl)alkanecarboxylic acid and 15% to 25% by weight of a diisocyanate component. The products contain relatively large amounts of organic solvents, and the corresponding coatings do not always satisfy the requirements in respect of application reliability and resistance properties.
EP-A 669 352 discloses aqueous dispersions of polyester-polyurethanes suitable for use in highly elastic paints, coatings and sealants, especially in soft-feel coating materials. The polyester-polyurethanes are substantially reaction products of linear polyester polyols and other linear polyols, such as polycarbonate diols or polyether diols, for example, hydroxy- and/or aminocarboxylic acids, if appropriate, low molecular weight compounds containing hydroxyl groups or amino groups, optionally hydrophilic alcohols containing ethylene oxide units, and a polyisocyanate component. The products are less suitable for paints and coatings, such as baking surfaces or one-coat topcoats, for example, owing to their high elasticity, the pronounced soft-feel effect and the low functionality (substantially difunctional). The products described in EP-A 669 352 are employed essentially in aqueous three-component paints composed of hydroxy-functional dispersion, non-functional dispersion and crosslinker. In terms of preparation and application, this system is much too complicated for numerous fields of application. Consequently there continues to be a need for cost-effective alternatives which are easily preparable, and which ought to exhibit improved application properties.
DE-A 3 901 190 describes a process for preparing aqueous, oxidatively drying alkyd resins (which, in other words, are crosslinkable by reaction with atmospheric oxygen), which contain 70% to 88% by weight of an alkyl resin, 4% to 7% by weight of 2,2-bis(hydroxymethyl)alkanecarboxylic acid and 10% to 23% by weight of a diisocyanate component and additionally contain organic solvents. These products are unsuitable for preparing binder combinations having the properties described above, as they exhibit very severe yellowing when cured at elevated temperatures, owing to the high fractions of unsaturated fatty acids.
Although the prior art covers a variety of polyester-polyurethane dispersions, there continues to be a need for improved, hydroxy-functional, polyester dispersions containing urethane groups. These dispersions ought to be easily preparable on the basis of inexpensive raw materials, in other words using minimal amounts of—generally expensive—hydrophilicizing agents and isocyanate components. Moreover, in their aqueous supply form, they ought to contain little organic solvent, ought to be stable for a relatively long time, in other words, not exhibit any disproportionately large hydrolytic degradation on storage, and in combination with a very wide variety of crosslinker resins ought to produce storage-stable paint formulations. These paint formulations ought to ensure high application reliability, particularly with respect to flow and susceptibility to cratering. Furthermore, they ought to exhibit coat thicknesses which can be achieved without defect, ought not to have any pronounced soft-feel character, and overall, after curing, ought to exhibit a balanced level of properties in respect of hardness/elasticity, adhesion, solvent resistance and film optical properties.
Surprisingly it has been found that special hydroxy-functional polyester dispersions containing urethane groups meet the requirements set out above and in combination with crosslinker resins, such as melamine resins and/or polyisocyanates, for example, can be cured to paints and coatings having the requisite level of properties. A particularly surprising feature is that even with the extremely low levels of hydrophilic groups for this class of raw material, it is possible to obtain extremely stable and relatively fine-particled dispersions. In spite of the use of large amounts of highly branched and hence highly functionalized polyesters, it is possible, with a reaction regime in accordance with the invention, to prepare stable dispersions, which meet the requirements described, in an inexpensive way.